The resin transfer molding (RTM) process is a manufacturing process for producing high quality composite structures in a cost-effective manner. Composite parts having complex geometries may be fabricated by this process in which liquid reactants are injected at low pressures in the range of about 275 to 414 kPa into a mold containing a fiber-reinforcement preform. The multiple plies of the fiber preform may be stacked inside the mold, factory-stitched or tacked together in some other fashion to provide precise fiber placement. The thermosetting, liquid polymer reactants of the resin system may be heated (to reduce viscosity) and injected through a static mixer into the mold, while vacuum evacuation of the preform is applied to reduce void formation and increase the resin transfer speed. When resin infusion is complete, the mold assembly is ramped to a cure temperature, dependent on the resin system, to effect a complete thermal cure of the composite part. Optionally, the composite part may be rapidly cured to an intermediate stage and removed from the mold, after which a free-standing post-cure is applied. This approach allows for more efficient mold use.
A major problem associated with prior art RTM resins is the difficulty of obtaining thermosetting matrices of high toughness that possess low viscosity at ambient temperatures. Tough resins such as liquid polybutadiene polymers and aliphatic, flexible chain epoxies lack the stiffness and operating temperature properties required for aerospace quality composite structures. Resin matrices such as polyester resin lack both toughness and capability for service at elevated temperatures, while other thermosetting matrix materials such as standard epoxies, polyimides, cyanate esters and bismaleimides either lack toughness or exhibit a high viscosity that necessitates processing at elevated temperatures. High process temperatures are undesirable because working life is reduced due to resin advancement; precise control of resin temperatures becomes more critical, reducing robustness of the process with more expensive, high temperature resistant mold and plumbing apparatus being required. The use of resin transfer molding in fabricating quality aerospace composite laminates that need high toughness has previously been accomplished only by using high viscosity thermoset or thermoplastic polymers and processing them at elevated temperatures.
The resin compositions of this invention also have application as filament winding resins. Wet filament winding is an art long practiced in manufacture of rocket motor cases and other high performance composite structures. Important in this art is selection of an appropriate winding resin for use in providing the matrix of the resultant composite structures containing high strength, continuous filaments in a form such as rovings, tows or bundles of glass, carbon, aramid, boron or like fiber. The fiber reinforcement preforms used in the RTM process are made from any of the aforementioned fibers.
The selection of an appropriate winding resin needs to consider a number of factors including the type of body being wound, e.g. size of the body, complexity in the shape of the body as well as the desired mechanical properties of the final cured composite. Generally, although desired mechanical properties alone would seem to drive selection in high performance applications, these final properties can be governed to a large degree by how well the winding resin is adapted to the filament winding application.
Development of desirable winding resins for wide applicability in making high performance composites requires an artful selection among a number of competing factors, particularly with respect to provision of winding resins that have sufficiently wide windows of processability for general application to a number of winding situations. For example, the winding resin needs sufficiently low viscosity for extended periods to enable consistent and thorough impregnation of the rovings, tows or bundles prior to completion of the winding operation. But the viscosity should be not so low as to permit resin migration after the rovings, tows or bundles are wound on the mandrel or other body shaping the filaments into desired form for the composite structure. The winding resin also needs to have adequate working life so that the body being wound has sufficient tack to accept and retain precise placement of subsequently applied rovings, tows or bundles in completing the filament wound body. Still other factors of adequate winding resins include appropriate gel time at ambient temperature, i.e. the time after winding after which the resin provides some integrity in holding high strength filaments in precise relation to their application, as well as controlled viscosity changes during heating used for curing the wound preform. A satisfactory winding resin therefore has many of the same properties as those desired in a resin for the RTM process.
Winding resins, even having an acceptable compromise of the foregoing and other factors, need also to provide a final cured body with adequate high and low temperature mechanical properties, particularly tensile strength, as well as, in the case of pressure vessels serving as rocket motor cases, desired strength at high pressure and high toughness to improve reliability in the field.
Resinous epoxy compounds formulated with curing agents such as primary amines have been found useful in meeting many of the foregoing criteria so as to serve, when properly formulated, as desirable winding resin systems. However, a long standing problem with these formulations has been the difficulty in achieving high toughness in low viscosity (&lt;2000 cps) resin systems. The challenge has been to develop appropriately low viscosity and desired wetting characteristics of the formulated resin prior to winding so as to allow adequate impregnation of the rovings, tows or bundles, coupled with acceptable gel time, working life and other criteria, in combination with providing adequate mechanical properties and high impact resistance to the resulting composite wound body after curing.
High viscosity resinous epoxy compounds having high molecular weight per epoxy group are known for their ability to provide toughness and elevated temperature resistance to winding resins that cure into high strength filament wound bodies. Lower viscosity winding resins comprising multi-functional resinous epoxy compounds having a lower molecular weight per epoxy group are known in providing advantageously better wetting characteristics at lower processing temperature than high viscosity resins; however, these low viscosity resinous epoxy compounds yield filament wound composite structures with lower strength, particularly at elevated temperatures. The foregoing dichotomy has been mitigated to some degree by heating high viscosity winding resins prior to their impregnation of the rovings, tows or bundles of high strength filaments used in filament winding, and then winding these filaments on the body shaping the uncured composite. However, such heating adds unwanted complexity to filament winding operations and undesirably advances the winding resin.
Low viscosity winding resins have been described, for example, in U.S. Pat. No. 4,255,302 to Adams, et al. This patent discloses a composition of the diglycidyl ether of bisphenol A, a diglycidyl ester of linoleic dimer acid, a diglycidyl ether of butanediol, and an aromatic amine curing agent. The fiber used with the disclosed composition is Kevlar.RTM. polyarylamide fiber.
U.S. Pat. No. 4,778,851 to Henton et al. discloses epoxy resin compositions that have been toughened by including as a dispersed phase rubber particles having a grafted shell which is cocurable with the epoxy resin. Crosslinked acrylic rubber cores are preferred. Polyepoxides are preferably glycidyl ethers of polyhydric alcohols including bisphenol A.
Another composition of epoxy resins useful as a matrix resin in combination with fiber is disclosed in U.S. Pat. No. 4,515,912 to Sayles. U.S. Pat. No. '912 describes a low shrink resin which includes a blend of bisspiroortho carbonate, an amine curing agent which is o-phenylenediamine boron trifluroide etherate, butanediol diglycidyl ether, the diglycidyl ether of bisphenol A and the epoxidized dimer of oleic acid.
U.S. Pat. No. 4,101,693 to Tsen, et al. discloses low viscosity, epoxy precursor resins made from a combination of an epoxy resin of a diglycidyl ether which is the reaction product of bisphenol A and epichlorohydrin having an equivalent weight of 170 to 200 and an average functionality of not more than two epoxy groups per molecule with a diglycidyl ether of bisphenol A of equivalent weight of 600 to 1600 having not more than two epoxy groups per molecule.
U.S. Pat. No. 4,309,473 to Minamisawa, et al. discloses high viscosity, prepreg epoxy resins, where the fiber strand is a high strength fiber and the resin comprises a thermo-setting resin such as an epoxy resin of bisphenol A and epichlorohydrin having a softening point of 60.degree. C. or less in combination with an epoxy resin having an average molecular weight of about 10,000 or more.